campaigns: display a banner when previewing a campaign that would require missing user tokens

Created by: LawnGnome

The scenario: a user is about to apply a campaign that pushes to GitHub, but the user does not have a token for GitHub in the Sourcegraph database.

We need to: display a banner in the UI indicating the problematic code hosts and providing a call to action. (Initially, pasting a token. Eventually, provisioning a token via OAuth.)

This will need to come back from the backend somehow: whether this is better achieved through extending our existing GraphQL queries (by adding a field for campaign notices, or something like that) or adding a new one is as yet undetermined.